Abstract

Secondary pneumonia due to Staphylococcus aureus (S. aureus) causes significant morbidity and mortality. The aim of the research was designed
a novel DNA vaccine encoding the Mycobacterium tuberculosis secreted antigen Ag85A fused with the influenza A virus (IAV) HA2 protein to provide
protection against both influenza and secondary infection with S. aureus. The DNA vaccine vector efficiently expressed the encoded antigen in mammalian cells,
as determined by RT-PCR, Western blotting and immunofluorescence analysis. Mice were
immunized with the vaccine by intramuscular injection before challenge with IAV and
S. aureus. The pulmonary and the splenocyte culture IFN-γ levels were significant higher in
immunized mice than their respective controls. Although the antibody titer in the
HI test was low, the sera of mice immunized with the novel vaccine vector were effective
in neutralisation assay in vitro. The vaccine could reduce the loss of body weight in mice during IAV challenge. Both
Western blotting and RT-PCR showed that the vaccine markedly enhanced toll like receptor
2 (TLR2) expression in splenocytes after the secondary infection with S. aureus. The survival rate of mice with high TLR2 expression (pEGFP/Ag85A-HA2 or iPR) was
significantly increased compared with mice immunized with pEGFP/HA2 after challenge
with S. aureus. However, the pulmonary IL-10 concentration and S. aureus titer were significantly decreased in immunized mice, and expression of TLR2 was
increased after challenge with S. aureus. These results demonstrated that Ag85A could strengthen the immune response to IAV
and S. aureus, and TLR2 was involved in the host response to S. aureus.